I can confirm that the PCB markings are correct.

On a working EP, with the ground clip of an oscilloscope probe connected to the right side of C2 (0V) and the tip connected to the left side of C2, there is approx -1v across the capacitor ie. the right side is more positive than the left side. Except for a few ms at powerup which might explain why C2 is a common failure.


Here purple is 12V,  blue is 5V and yellow is across C2, at power on (note different v/div for each channel.)

I was looking at power on because I thought the purpose of C2 might be to delay 12V coming up so that 5V comes up first. But here you can see that the 12V trace reaches 9V almost immediately and then goes to 12v ~3ms later, so I'm still not 100% sure!

I'm quivering with anticipation, waiting for his reply!

I did a search on forum, looking for all references to "C9", but it didn't turn up any satisfactory explanation.

Same here.
All available (different) schematics show the same direction.

It is more of a habit. I usually replace every 7805 in old computers either with the OKI-78SR or the TSR 1-2450 in order to reduce the heat. I generally prefer the TSR 1-2450, but they don't fit in the EP without removing the cooler and that would leave an ugly hole.

Thanks for this. I suspect there is an issue with one of the 7805s on my enterprise and was planning to replace both with new 7805s but will use OKI-78SR now.

Restless Andre

Mit gondolsz miert rendeltem?